Sex Ratios and Gamete Production Vary Across Cryptic Coral Lineages and Thermal Environments

Abstract Corals exhibit diverse sexual strategies that can shape their reproductive ecology and responses to environmental challenges. Thermal anomalies can impair coral reproduction, having broad implications for population persistence. Cryptic genetic diversity can also influence coral resilience to warming oceans, yet few studies have addressed lineage reproductive biology. In gonochoric species, where individuals produce only sperm or eggs, thermal anomalies may differentially affect sexes due to different physiologies and unequal energetic investment in gametes. Here, sex ratios, fecundity, and spatial distributions are assessed in two lineages of Siderastrea siderea spanning six sites in Panamá. Differing in thermal variability, these sites offer a natural gradient to explore reproductive variation between lineages across environments. Histology data suggest sex bias towards sperm-producing colonies across most sites in both lineages, with no egg-producing colonies observed at two sites. Lineage-specific reproductive patterns show evidence for a mixed reproductive strategy (gonochorism, hermaphroditism) in one lineage, while the other exhibited simultaneous hermaphroditism. Preliminary spatial analyses indicate non-random distributions of sex types across sites, with spermatocyte-producing colonies more closely neighboring oocyte-producing colonies than expected by chance. Lastly, sites that experienced the greatest thermal anomalies in 2023/2024 had the largest sex bias, suggesting these thermal anomalies may alter colony sex expression. Together, these findings underscore the importance of incorporating cryptic diversity into assessments of coral reproductive potential and provide a valuable baseline for understanding how reproductive traits are shaped by changing environments.